The present invention relates generally to photolithographic techniques for micromachining. More particularly, the present invention relates to a method for making positive (extending above a substrate surface) and negative (extending into the substrate surface) mechanical features. Both positive and negative features are defined according to a single mask step.
Micromachined chips are commonly used in the fiber optics industry to build microoptical submounts. Such chips typically have etched pits for holding lenses, fibers, or active optical devices. Micromachined chips may also have posts for aligning fibers, lenses or active devices (e.g. lasers, photodetectors).
FIG. 1 shows an exemplary optical submount known in the art of microoptics. The submount has posts 10 (positive features) and V-grooves 12 (negative features). Optical fibers are disposed in the V-grooves for alignment and an optoelectronic chip is butted against the posts 10 for alignment. Together, the V-grooves 12 and posts 10 provide for accurate relative alignment between the optical fibers and the optoelectronic chip.
For the optoelectronic chip and the optical fibers to be accurately aligned, the V-grooves and posts must be accurately aligned. Accurate alignment between the posts 10 and V-grooves 12 can be difficult to assure because the posts and V-grooves are typically fabricated using different processes defined in different masking steps. Alignment between the V-grooves and posts is limited by the accuracy of mask alignment. For example, a misalignment of a V-groove mask to the posts results in a misalignment of the V-grooves with respect to the posts. Also, V-groove-post alignment is limited by problems associated with performing lithography on nonplanar surfaces.
Also, it is useful to provide metal patterns that are accurately aligned with the posts and V-grooves.
The present invention provides methods for making a micromachined device having a positive feature (e.g., a post) and a negative feature (e.g. a pit)-. In the present invention, a wafer having a device layer, an etch stop layer and a handle layer (e.g., a silicon-on-insulator, or SOI, wafer) is used.
In a first method, a patterned hard mask is disposed on top of the device layer. The device layer is directionally dry etched (e.g. by RIE or DRIE) to expose the etch stop layer in areas not covered by the hard mask. This forms a positive feature 26. Next, metal is deposited on areas of the etch stop layer exposed by the directional dry etching. Then, portions of the device layer are removed, leaving the positive feature in place (e.g. a post), and leaving the metal on the etch stop layer.
Then, optionally, the metal patterns on the etch stop layer are used as a mask to etch through the etch stop layer and expose areas of the handle layer. Then, the handle layer is etched to form negative micromachined features. The positive and negative features are defined according to the same single mask step. The handle layer can be etched using dry etching or wet (e.g. anisotropic) etching.
The positive feature can be oxidized before the handle layer is etched so that the positive feature is protected during the handle layer etch.
Alternatively, the etch stop layer is removed from areas where the device layer is etched away in the dry etching step. In this way, the device layer acts as a mask for patterning the etch stop layer. Then, the etch stop layer acts a mask to etch the handle layer. Optionally, metal is also deposited on the etch stop layer in areas where the device layer is removed.
The present invention also includes an embodiment where a metal layer is patterned on the device layer and etch stop layer after the device layer is removed. Then, the metal layer is used as a mask to etch (e.g. dry etch or wet etch) the device layer. Similarly, the metal layer is used to pattern the etch stop layer; the etch stop layer then acts as a hard mask for etching the handle layer.
The present invention also includes an embodiment where a hard mask is patterned on the device layer, and a removable mask (mask 100) is deposited on top of portions of the hard mask. Directional dry etching is performed to expose the etch stop layer. Metal is deposited on exposed areas of the etch stop layer. Then, the device layer is etched with an orientation dependent etchant (e.g. KOH or EDP on silicon).
The present invention also includes an embodiment where the device layer and etch stop layer are removed from the same area, exposing the handle layer. Then, a dielectric coating is applied to the handle layer, and a metal pattern is deposited on the dielectric coating on the device layer and on the handle layer. Then, the device layer is directionally dry etched, forming the positive feature. The handle layer is processed according to previous embodiments to form the negative feature. The dielectric coating provides electrical isolation between the metal pattern and the handle layer.
The present invention also includes a micromachined apparatus having a handle layer with a negative etched feature, an etch stop layer having a hole above the negative feature, a metal pad disposed on the etch stop layer, and a positive feature disposed on the etch stop layer. The metal pad has an inner portion adjacent to the hole, and an outer portion disposed opposite the hole. The inner portion has surface damage or surface modification not present on the outer portion. The surface modification is characteristic of etching processes used to make the hole or the negative feature. Such surface modification includes pitting, sputtering/redeposition of metal, material removal, fluoropolymer deposition, and chemical erosion.